Photoelectric linear image sensor having multilayer insulating layer to reduce defects

ABSTRACT

An improved linear image sensor of the type having a plurality of linearly arranged photodetecting resistors of amorphous silicon film, leader electrodes individually connected thereto, striped matrix electrodes, and an insulating layer covering the striped matrix electrodes is provided. Each of said leader electrodes are in contact with each of said striped matrix electrodes via a hole formed by patterning in the insulating layer. The insulating layer is made up of a plurality of insulation films laminated one over another by repeating the steps of film forming and patterning. Thus, even though pinholes occur during deposition or patterning of individual layers, there is only a small probability that two or more pinholes will occur at the same place. The risk of short circuits caused by pinholes is therefore substantially reduced.

BACKGROUND OF THE INVENTION

The present invention relates to a contact image sensor havingphotoconductive detecting elements of amorphous silicon (a-Si) arrangedin a linear array of approximately the same length as the object to beread.

A circuit for a conventional linear image sensor for matrix reading isshown in FIG. 2. The linear image sensor 10 is made up ofphotoconductive detecting elements 1 which are linearly arranged at adensity of 8 elements, 12 elements, or 16 elements per millimeter. Thetotal number of detecting elements in a linear image sensor for A-4 sizepaper is 1728 if it has 8 elements per millimeter. The 1728 elements aredivided into 32 groups, each consisting of 54 elements. Positiveelectrodes of the 54 elements as a group are connected to first switches20 in common. Negative electrodes at the corresponding positions in eachgroup of the 54 elements are connected to second switches 30 in common.Therefore, the number of first switches 20 is 32 and the number ofsecond switches 30 is 54. First switches 20 are connected to secondswitches 30 via a power source 40 (6-10 V) and a current detector 50. Toachieve reading with this linear image sensor, the resistance of theindividual elements 1 is measured consecutively by turning on and offsecond switches 30 sequentially, with one group of first switches 20closed. If one of the detector elements 1 receives light, it decreasesin resistance, permitting a current to flow through the detector 50.Without light, the detector element has a high resistance and prevents acurrent from flowing through the detector 50. Thus the detector elements1 distinguish black parts from white parts on the original across whichthe linear image sensor 10 is passed.

A conventional linear image sensor based on amorphous silicon isconstructed as shown in FIG. 3. There is shown a soda glass substrate 2which is coated with an SiO₂ film 21 to hold back sodium. On thesubstrate 2 are formed matrix metal electrodes 31, 32, 33, 34 ofchromium. (Only four electrodes are shown.) They are covered with asilicon nitride (SiN) film 4. These matrix electrodes are connected tosecond switches 30 shown in FIG. 2. On the substrate 2 is also formed anintrinsic amorphous silicon (a-Si) film 51. The intrinsic a-Si film 51is connected to a metal electrode 6 via an n-type a-Si film 52. Themetal electrode 6 is connected to switches 20 shown in FIG. 2. Theintrinsic a-Si film 51 is connected also to metal electrodes 61, 62, 63,64 via an n-type a-Si film 52. The lead electrodes 61, 62, 63, 64 areconnected to the matrix electrodes 31, 32, 33, 34, respectively, via thecontact hole 41 formed in the SiN film 4. The array of the photosensorsis covered with a transparent protective film 71 of SiN or epoxy resin,and the matrix circuit is covered with an opaque protective film 72.

The conventional image sensor as mentioned above has a weak pointarising from the fact that the leader electrodes 61, 62, 63, 64 comeinto contact with the matrix electrodes 31, 32, 33, 34, respectively, atas many as 1728 places in the case of an image sensor for A-4 sizepaper. Thus, the image sensor becomes defective even if it has only oneshort circuit caused by a pinhole defect in the insulation film 4. Forthis reason, the defective fraction of conventional image sensors isinvariably higher than 50%. It is thus an object of the presentinvention to solve the above-mentioned problem and to provide a linearimage sensor at a low fraction defective.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a linear imagesensor of the type having a plurality of linearly arrangedphotodetecting resistors of amorphous silicon film, leader electrodesindividually connected thereto, striped matrix electrodes, and aninsulating layer covering the striped matrix electrodes. Each of saidleader electrodes are in contact with each of said striped matrixelectrodes via a hole formed by patterning in the insulating layer. Theinsulating layer is made up of a plurality of insulation films laminatedone over another by repeating the steps of film forming and patterning.

Thus, even though pinholes occur during deposition or patterning ofindividual layers, there is only a small probability that two or morepinholes will occur at the same place. The risk of short circuits causedby pinholes is therefore substantially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a plan view and FIG. 1(b) is a sectional view taken alongthe line A--A in FIG. 1(a), both showing the linear image sensor in oneembodiment of the present invention.

FIG. 2 is a circuit diagram illustrating how a conventional contact-typelinear image sensor performs matrix reading.

FIG. 3 is a conventional linear image sensor based on amorphous silicon.FIG. 3(a) is a plan view, and FIG. 3(b) is a sectional view taken alongthe line B--B in FIG. 3(a).

FIGS. 4(a), 4(b), 4(c) and 4(d) are sectional views illustrating thesteps of producing the sensor as shown in FIG. 1.

FIG. 5 is a graph showing the relationship between the photoconductivityand the amount of phosphorus added to the photoconductive a-Si film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of the invention is explained with reference to FIG. 1(a) andFIG. 1(b). The former is a plan view and the latter is a sectional viewtaken along the line A--A in FIG. 1(a). In FIG. 1 and FIG. 3, likereference characters designate like or corresponding parts. With regardto FIG. 1, a soda glass substrate 2 is provided, which need not becoated by a SiO₂ film as in the prior art structure. On the substrateare formed metal matrix electrodes 31, 32, 33, 34 of chromium. Thematrix electrodes 31, 32, 33, 34 are covered with a silicon nitride(SiN) film 4, and the SiN film 4 further extends beneath the entirephotosensor array. The SiN film 4 is covered with SiC film 8, which isin turn covered with an a-Si film 5. The a-Si film 5 is connected to ametal electrode 6 and to the leader electrodes 61, 62, 63, 64 via ann-type a-Si film 52. The common metal electrode 6 is connected toswitches 20 as shown in FIG. 2. The leader electrodes 61, 62, 63, 64 areconnected to the matrix electrodes 31, 32, 33, 34 respectively, throughthe contact hole 41 which is formed through the several layers formedover the matrix electrodes, i.e., the SiN film 4, the SiC film 8, thea-Si film 5, and the n-type a-Si film 52. The a-Si film 5 and the n-typea Si film 52 are disposed over the entire photosensor array, and arebeneath the leader electrodes 61, 62, 63, 64. The entire array is thencovered by a transparent protective film 71 of SiN or epoxy resin, whichis extended to also cover the matrix electrodes. The matrix electrodesare further covered with an opaque protective film 72 above thetransparent protective film 71.

The linear image sensor described above can be produced according to thesteps shown in FIGS. 4(a) to 4(d). First, a glass substrate 2 is coatedwith a metal such as chromium to a thickness of 1000 to 5000 Å byevaporation or sputtering. Striped matrix electrodes 31, 32, 33, 34 areformed by photolithography. On the striped matrix electrodes is formed aSiN film 4 (1000 Å to 1 μm thick) by glow discharge decomposition of a1:3 gaseous mixture of ammonia (NH₃) and silane (SiH₄). The SiN film 4is patterned using SF₆ gas by photolithography to make the connectinghole 41. (See FIG. 4(a).)

Then, on the SiN film 4 a SiC film 8 (1000 to 5000 Å thick) is formed byglow discharge decomposition of silane containing 20-80% of acetylene(C₂ H₂). On the SiC film 8 are further formed, one over the other, ana-Si film 5 (1000 to 5000 Å thick) by glow discharge decomposition ofsilane containing 100 ppm or less of phosphine, and an n-type a-Si film52 (100 to 500 Å thick) by glow discharge decomposition of silanecontaining 0.5 to 2% of phosphine. The laminated films are patternedusing SF₆ gas or CF₄ gas containing oxygen by photolithography to makethe extension for the connecting hole 41. (See FIG. 4(b).)

In another embodiment of the present invention, the patterning of theSiN film 4 was followed by the deposition of an SiN film (1000 to 5000 Åthick) in place of the SiC film 8. The subsequent steps were carried outin the same manner as described above to form the a-Si film 5, then-type a-Si film 52, and the extension of the connecting hole 41.

In a further embodiment of the present invention, both of the insulatingfilms 4 and 8 were SiC films. The SiC films were formed by glowdischarge decomposition of a mixed gas of C₂ H₂ and SiH₄, with themixing ratio adjusted so that the C/Si ratio was 10 to 30% (The C₂ H₂may be replaced by methane, ethane, propane, and other hydrocarbons).The patterning (etching) of the SiC film by photolithography was carriedout by using SF₆ gas or CF₄ gas containing several percent of oxygen.

On the multi-layer films formed as described above were formed thecommon electrode 6 and the individual leader electrodes 61, 62, 63, 64from chromium or other metals by sputtering or electron beamvaporization, followed by patterning by photolithography. Using the thusformed electrode pattern as a mask, the n-type a-Si film 52 was etchedin a plasma of SF₆ gas or CF₄ gas containing oxygen. (See FIG. 4(c).)

Finally, on the electrodes were formed a transparent protective SiN film71 by plasma decomposition of a silane-ammonia mixed gas, using a maskwhich permits the end of the common electrode 6 and the respective ends(not shown) of the matrix electrodes 31, 32, 33, 34 to be exposed forconnection to the outside circuits. The transparent protective SiN filmwas coated, followed by baking, with an opaque epoxy resin by printingto form the protective film 72. (See FIG. 4(d).)

In the above-mentioned embodiments, the insulating layer of the matrixelectrodes was formed by repeating the film formation and patterningtwice each. As the result, the fraction defective attributable to shortcircuits in the matrix circuits was reduced. With the a-Si film 5 andtransparent protective film 71 extended above the matrix circuits, theinsulation and protection of the matrix circuits are enhanced and thisleads to the decrease in the number of defectives to below 20%. Theabove-mentioned process can be carried out without any substantialincrease in steps because the SiC film 8 or the second SiN film can beformed simply by replacing the gas in the same CVD apparatus used forthe formation of the SiN film 4 or the a-Si films 5 and 52. Moreover, inthe above-mentioned examples, the SiN insulation film 4 is extendedunderneath the photosensor array. This makes it unnecessary to cover thesubstrate 2 of soda glass with an SiO₂ film because it holds backsodium.

In the above-mentioned examples, the a-Si film 5 was incorporated with atrace amount of phosphorus to increase the sensitivity of thephotosensor. FIG. 5 shows the relationship between the amount ofphosphorus (in terms of phosphine gas (in ppm) in silane gas used forthe formation of the a-Si film 5) and the electric current induced by abias voltage of 1 V under irradiation (100 lx) from a light emissiondiode (wavelength 565 nm). The broken line 12 represents the darkcurrent. It is noted that the addition of a small amount of phosphorusis effective in the increase of sensitivity. It is also noted that thedark current increases with the amount of phosphorus doped. With theamount of phosphorus doped in the range of 5 to 70 ppm, preferably 10 to50 ppm, the sensor has an increased sensitivity, giving a sufficientlyhigh ratio of signal current to dark current. Thus the present inventionovercomes one of the disadvantages involved in the conventionalproducts, namely that the signal current is too small (about 10 nA under1000 lx) to process without difficulties.

The insulating film of SiC is advantageous over the insulating film ofSiN in that it keeps a stable surface when a photoresist is applied andit is etched accurately. This advantage leads to a decrease in fractiondefective. In the embodiment in which the SiC films were formed in twolayers, the fraction defective was 17%.

Despite the fact that the SiC film has slight photoconductivity, theimage sensor of the present invention has good characteristics onaccount of the opaque protective film 72 which shields the incidentlight. If there is a possibility of light entering through the glasssubstrate 2, it is desirable to shield the glass surface opposite to thematrix electrodes.

The number of the insulating films laminated on top of the other is notlimited to two. The greater the number of the insulating films, the morecertain it is to prevent short circuiting caused by overlapped pinholes,although the number of steps increases. The effective insulating filmmay be made of SiO₂ and other materials.

The linear image sensor of the present invention is characterized inthat multi-layer insulation films formed by repeating evaporation andpatterning separate the leading electrodes of linearly arranged a-Siphotodetecting resistors from the matrix electrodes connected to theirrespective switches. This structure contributes to reducing the fractiondefective arising from the short circuits in the insulating layer,without substantial increase in production steps.

I claim:
 1. A linear image sensor comprising a substrate having disposedthereon a common electrode and a linear array of a plurality ofdetecting elements, each detecting element comprising(a) aphotodetecting resistor in electrical contact with said commonelectrode; (b) a matrix electrode adjacent to the photodetectingresistor, said matrix electrode being adapted for connection to anexternal circuit; (c) an insulating layer disposed over a portion ofsaid matrix electrode and having a window formed therein coincident withsaid matrix electrode; and (d) a leader electrode disposed over saidinsulating layer, said leader electrode making electrical contact withsaid matrix electrode through the window in the insulating layer andfurther making electrical contact with the photodetecting resistor, butnot making direct electrical contact with said common electrode, whereinthe insulating film comprises a plurality of separately formed layers,one over another.
 2. A linear image sensor as claimed in claim 1,wherein the photodetecting resistors comprise a film of n-type amorphoussilicon.
 3. A linear image sensor as claimed in claim 2, wherein thefilms of the insulating layer comprise materials selected from the groupconsisting of amorphous silicon, silicon nitride and silicon carbide. 4.A linear image sensor as claimed in claim 2, wherein the amorphoussilicon film constituting the photodetecting resistors contains 5 to 70ppm of phosphorous.
 5. A linear image sensor comprising a substratehaving disposed thereon a common electrode and a linear array of aplurality of detecting elements, each detecting element comprising:(a) aphotodetecting resistor in electrical contact with said commonelectrode; (b) a matrix electrode adjacent to the photodetectingresistor, said matrix electrode being adapted for connection to anexternal circuit; (c) an insulating layer disposed over a portion ofsaid matrix electrode and having a window formed therein coincident withsaid matrix electrode; and (d) a leader electrode disposed over saidinsulating layer, said leader electrode making electrical contact withsaid matrix electrode through the window in the insulating layer andfurther making electrical contact with the photodetecting resistor, butnot making direct electrical contact with said common electrode, whereinthe insulating film comprises a plurality of separately formed layers,one over another; and wherein the photodetecting resistor is one layerof the insulating layer and extends over the matrix electrode.
 6. Thelinear image sensor of claim 5, wherein the photodetecting resistorcomprises a film of n-type amorphous silicon.
 7. A linear image sensoras claimed in claim 5 wherein a transparent protective film is disposedabove the entire leader electrode.
 8. A linear image sensor as claimedin claim 6, wherein one or more layers of the insulating film inaddition to the n-type amorphous silicon film extend beneath the leaderelectrodes.